lens

Focal length and angle of view

A lens is described first by its focal length, the distance from its optical center to the point where it focuses parallel rays, conventionally stated in millimeters. Focal length, together with the dimensions of the recording medium, determines the angle of view — the extent of a scene the lens projects onto the film or image sensor. A short focal length yields a wide angle of view and is termed a wide-angle lens; a long focal length yields a narrow angle and is termed a long-focus lens, often called a telephoto after the compact design commonly used for such lenses. A normal lens produces an angle of view roughly comparable to human central vision for a given format, an effect that occurs when the focal length approximates the diagonal of the frame. Because angle of view depends on the format, the same focal length frames a scene more tightly on a smaller sensor than on a larger one, a relationship that also bears on the aspect ratio of the captured image.

Aperture, light, and depth of field

The aperture is the opening, usually an adjustable iris, that controls how much light passes through the lens. Its diameter is expressed as an f-stop or f-number — the ratio of the focal length to the effective aperture diameter — so that smaller f-numbers denote larger openings. Each full stop (for example, from f/2.8 to f/4) halves or doubles the light admitted, making the f-stop a principal control of exposure. The aperture also governs depth of field, the range of distances rendered acceptably sharp: a wide aperture (small f-number) produces shallow depth of field, while a small aperture (large f-number) extends it. Depth of field also varies with focal length and with the distance to the subject.

Prime, zoom, and anamorphic lenses

A prime lens has a single fixed focal length, whereas a zoom lens contains movable groups of elements that vary focal length over a continuous range. Primes are generally simpler optically and offer wider maximum apertures for a given size, while zooms trade some of that capability for the convenience of a variable angle of view. An anamorphic lens introduces a cylindrical element that optically squeezes a wide field horizontally onto the frame; in projection or processing the image is stretched back to its intended proportions, yielding a wide aspect ratio from a more nearly square area of film or sensor. Anamorphic optics also produce characteristic oval out-of-focus highlights and horizontal flares. Together with the recording medium, the lens thus shapes the field of view, the rendering of depth, and the proportions of the finished image, and its behavior is central to the practice of cinematography.